Pakistan's disaster management agency has said that the country's worst floods in its history have affected 12 million people.

Nadeem Ahmed, chairman of the National Disaster Management Authority, said that figure only applied to the northwestern Khyber Pakhtunkhwa and central Punjab provinces.

Figures from the southern Sindh Province were not yet available.

Strong rainstorms have swollen the Indus River -- raising concerns about severe flooding during the next 48 hours in the densely populated southern province of Sindh.

The deputy director of the UN's World Food Program, Amir Abdulla, told a news conference in Rome today that the floods have wiped out much of the infrastructure in the affected areas, hampering assistance.

"To reach the people who are in great need of life support and life-saving assistance at this time -- food, medical , shelter -- the needs are huge -- and to get there, the World Food Program and our partners are using every means possible," Abdulla said.

Pakistani authorities issued a red alert for extreme flooding in the agricultural south, with a half million people being evacuated.

The United States has pledged more than $35 million in emergency aid to help Pakistan cope with its worst flooding in 80 years.